Conventionally, an electrophotographic image forming apparatus will experience problems in producing an image, such as a decline in image density or loss of image, if the developer (toner) falls short during operation. For this reason the apparatus usually is equipped with means for sensing amount of remaining toner in a developing unit and, when toner runs out, for presenting an indication of this fact to alert the user. The toner may thus be replenished before faulty images are produced. A capacity sensing method is known as one example of means for sensing amount of remaining toner.
Examples of an electrophotographic image forming apparatus include electrophotographic copiers, electrophotographic printers (e.g., LED printers and laser printers, etc.) and electrophotographic facsimile machines. Further, there are two types of process cartridges. In one type, at least one means from among corona discharge means, developing means and cleaning means is integrated with an electrophotographic photosensitive body to form a cartridge and the cartridge is adapted so that it can be installed removably in the main body of the electrophotographic image forming apparatus. In the other type, at least developing means and an electrophotographic photosensitive body are integrated to form a cartridge and the cartridge is adapted so that it can be installed removably in the main body of the electrophotographic image forming apparatus.
FIG. 2 illustrates one example of a developing unit for sensing amount of remaining toner according to the conventional capacity sensing method. As shown in FIG. 2, the developing unit has a developing blade 24, which is a developer regulating member, and a developing sleeve 21, which is a developer carrier. Disposed in the vicinity of the developing sleeve 21 substantially in parallel with the sleeve is a plate antenna PA, which is an electrode for sensing electrical conductivity. Amount of remaining toner is sensed by adopting a change in amount of toner between the developing sleeve 21 and antenna PA as a change in electrostatic capacity. To sense the electrostatic capacity, a developing bias comprising an oscillating voltage obtained by superimposing an AC voltage and a DC voltage is impressed upon the developing sleeve 21 and a current that flows between the plate antenna PA and ground at such time is converted to a DC voltage by a sensing circuit, thereby allowing the current to be read.
An image forming apparatus that has become available in recent years has a plurality of print modes in order to obtain the best image regardless of the type of printing medium (e.g., high-resistance paper such as thick paper), which can be of multifarious types. The setting of developing bias, setting of transfer bias and fixing conditions, etc., are changed over in accordance with the plurality of print modes to obtain a high image quality that conforms to the type of printing medium. In an image forming apparatus of this kind, it is possible to obtain an effective improvement in image quality especially by changing over the frequency of developing bias in the developing bias settings. It should be noted that development is carried out using a developing bias that generally is obtained by superimposing a DC voltage on an AC voltage. The frequency of the developing bias is the frequency of the AC component of the developing bias.
However, in an image forming apparatus having remaining toner sensing means that uses developing bias applied to a developing sleeve as described above, a problem which arises is that the voltage value detected based upon the frequency of the developing bias differs in a case where the frequency of the developing bias is changed over in dependence upon the type of printing medium.
For example, in an arrangement having two print modes conforming to the printing media, assume that the frequency of developing bias in mode 1 is 2.0 kHz and that in mode 2 is 2.4 kHz. FIG. 7 illustrates the relationship between remaining amount of toner and detected voltage in each of these modes.
If output voltage for sensing of remaining amount of toner exceeds 3 V when the frequency of developing bias in mode 1 of the two print modes conforming to the printing media is 2.0 kHz, a faulty image will be produced owing to insufficient toner. Accordingly, if the voltage for rendering a decision to the effect that no toner remains is set to 3 V, a proper decision can be rendered in a case where the frequency of the developing bias is 2.0 kHz. However, in the case of mode 2 where the frequency is 2.4 kHz, the value of 3 V is not attained, image formation is carried out regardless of insufficient toner and, as a consequence, a faulty image is produced.
Conversely, if the voltage for rendering a decision to the effect that no toner remains is set to 2.7 V as the no-toner state at the frequency of 2.4 kHz in mode 2, a no-toner decision will be rendered, despite the fact that much toner remains, in a case where the frequency of the developing bias is 2.0 kHz.
Thus, in a case where image formation is carried out upon changing over the image forming mode by changing the frequency of the developing bias, the output value obtained in sensing amount of remaining toner will differ in dependence upon the frequency of the developing bias that has been set and, consequently, a problem which arises is that the amount of remaining toner cannot be sensed accurately.
In other words, the ability to sense amount of remaining developer accurately irrespective of the frequency of developing bias is required in an electrophotographic image forming apparatus having an image forming mode in which the frequency of an oscillating voltage prevailing at the writing of an image can be changed over in accordance with the type of printing medium.